Characterization of an FcγRI-binding peptide selected by phage display

The high-affinity IgG receptor, Fc gamma receptor I (Fc gamma RI), is expressed exclusively on myeloid cells, and there is a great interest in the targeting of vaccine antigens to Fc gamma RI using anti-human Fc gamma RI antibodies or fragments derived from such molecules. In order to reduce the size and complexity of the targeting reagent, we have searched for Fc gamma RI binding peptides in peptide libraries displayed on phage. The human monocytic cell line U937 was used as target. Phages that displayed the consensus peptide CLRSGXGC were selected and revealed increased binding to IFN-gamma stimulated versus non-stimulated U937 cells as well as to Fc gamma RI transfected versus non-transfected IIA1.6 cells. Furthermore, they bound the extracellular domains of soluble Fc gamma RI, but neither Fc gamma RIIA, Fc gamma RIIB nor Fc gamma RIIIB. Binding was inhibited by a synthetic version of the peptide, whereas neither human IgG nor the Fc gamma RI-specific monoclonal antibodies (mAb) mAb22 and 32.2 interfered. Flow-cytometry analysis and internalization studies showed that a synthetic biotin-conjugated peptide ADGACLRSGRGCGAAK-bio was able to target U937 cells and Fc gamma RI transfected IIA1.6 cells, and further to promote internalization and vesicular degradation of streptavidin coupled to 1 mu m magnetic beads. These peptides may have potential as Fc gamma RI targeting reagents.